Such fastening is generally provided by bars or legs secured to the engine and resting on the airframe, by bars secured to the engine and to the airframe, and/or by a mechanical connection with a main transmission gearbox to which the engine is coupled.
The framework (or airframe) of a helicopter presents deformation resonance modes; each resonance mode can be excited when the resonant frequency corresponding to a resonance mode is close enough to an excitation frequency; this leads to deformation of large amplitude and to large forces that can damage the aircraft. Amongst the vibratory phenomena that can excite these resonance modes, there are the harmonics of the frequency of rotation of the lift and propulsion main rotor of the helicopter, and in particular the harmonic that corresponds to the number of rotor blades.